The purpose of this method is to make sure no single trade or single day of trading hurts has a significant impact on the account. Therefore, a trader knows that they will not lose more in a single trade or day than they can make back on another by adopting a risk maximum that is equivalent to the average daily gain over a 30 day period. (To understand the risks involved in forex, see "Forex Leverage: A Double-Edged Sword.")
By contrast, if you just traded 20 EUR, a loss would not significantly affect your account balance. It would provide you with the opportunity to learn from your experience and plan your next trade more effectively. With this in mind, limiting the capital you are prepared to risk to 5% of your account balance (or lower) will put you in a better position to continue trading Forex (and improving your technique) over the long term.
The foreign exchange market is where currencies are traded. Currencies are important to most people around the world, whether they realize it or not, because currencies need to be exchanged in order to conduct foreign trade and business. If you are living in the U.S. and want to buy cheese from France, either you or the company that you buy the cheese from has to pay the French for the cheese in euros (EUR). This means that the U.S. importer would have to exchange the equivalent value of U.S. dollars (USD) into euros. The same goes for traveling. A French tourist in Egypt can't pay in euros to see the pyramids because it's not the locally accepted currency. As such, the tourist has to exchange the euros for the local currency, in this case the Egyptian pound, at the current exchange rate.